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Merge pull request 'Document db_engine' (#399) from doc-0.8 into main

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Reviewed-on: https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/399
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Alex 2022-09-29 17:29:44 +02:00
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doc/book/reference-manual/configuration.md
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@ -9,6 +9,8 @@ Here is an example `garage.toml` configuration file that illustrates all of the
metadata_dir = "/var/lib/garage/meta"
data_dir = "/var/lib/garage/data"
db_engine = "lmdb"
block_size = 1048576
replication_mode = "3"
@ -71,6 +73,47 @@ This folder can be placed on an HDD. The space available for `data_dir`
should be counted to determine a node's capacity
when [adding it to the cluster layout](@/documentation/cookbook/real-world.md).
### `db_engine` (since `v0.8.0`)
By default, Garage uses the Sled embedded database library
to store its metadata on-disk. Since `v0.8.0`, Garage can use alternative storage backends as follows:
| DB engine | `db_engine` value | Database path |
| --------- | ----------------- | ------------- |
| [Sled](https://sled.rs) | `"sled"` | `<metadata_dir>/db/` |
| [LMDB](https://www.lmdb.tech) | `"lmdb"` | `<metadata_dir>/db.lmdb/` |
| [Sqlite](https://sqlite.org) | `"sqlite"` | `<metadata_dir>/db.sqlite` |
Performance characteristics of the different DB engines are as follows:
- Sled: the default database engine, which tends to produce
large data files and also has performance issues, especially when the metadata folder
is on a traditionnal HDD and not on SSD.
- LMDB: the recommended alternative on 64-bit systems,
much more space-efficiant and slightly faster. Note that the data format of LMDB is not portable
between architectures, so for instance the Garage database of an x86-64
node cannot be moved to an ARM64 node. Also note that, while LMDB can technically be used on 32-bit systems,
this will limit your node to very small database sizes due to how LMDB works; it is therefore not recommended.
- Sqlite: Garage supports Sqlite as a storage backend for metadata,
however it may have issues and is also very slow in its current implementation,
so it is not recommended to be used for now.
It is possible to convert Garage's metadata directory from one format to another with a small utility named `convert_db`,
which can be downloaded at the following locations:
[for amd64](https://garagehq.deuxfleurs.fr/_releases/convert_db/amd64/convert_db),
[for i386](https://garagehq.deuxfleurs.fr/_releases/convert_db/i386/convert_db),
[for arm64](https://garagehq.deuxfleurs.fr/_releases/convert_db/arm64/convert_db),
[for arm](https://garagehq.deuxfleurs.fr/_releases/convert_db/arm/convert_db).
The `convert_db` utility is used as folows:
```
convert-db -a <input db engine> -i <input db path> \
-b <output db engine> -o <output db path>
```
Make sure to specify the full database path as presented in the table above,
and not just the path to the metadata directory.
### `block_size`
Garage splits stored objects in consecutive chunks of size `block_size`